
We propose the concept of RIS-aided cell-free network to further improve the network capacity of the cell-free network with low cost and power consumption. The key idea is to replace some of the bss in cell-free network by the energy-efficient riss and deploy more riss in the system for capacity enhancement. In the proposed RIS-aided cell-free network, all bss and riss are simultaneously serving all users cooperatively. To the best of our knowledge, this is the first attempt to exploit ris in cell-free network to improve the network capacity.

For the proposed RIS-aided cell-free network in the typical wideband scenario, we formulate the joint precoding design problem at the BSs and RISs to maximize the wsr of all users to optimize the network capacity. Since the considered scenario is very general, i.e., multiple antennas, multiple bss, multiple riss, multiple users, and multiple carriers, most of the considered scenarios in existing works, such as single bs, single ris, single user, and single carrier, or some of them are mutiple, are all special cases of the considered scenario in this paper.

Inspired by the methods introduced in [1]} [2]}, we propose a joint precoding framework to solve the formulated wsr maximization problem. Specifically, the proposed framework is an alternating optimization algorithm, which can approximate the local optimal solution gradually. We first decouple the active precoding at the bss and the passive precoding at the riss via Lagrangian dual reformulation and fractional programming. Then, the decoupled subproblems are reformulated as the qcqp problems, respectively. By solving the subproblems alternatively, the wsr will converge to a local optimal solution. The simulation results show that riss can improve the cell-free network capacity significantly. Note that the proposed joint precoding framework can also serve as a general solution to maximize the wsr in most of the existing RIS-aided scenarios in the literature.

